Optical device including multilayer reflector and vertical cavity surface emitting laser

ABSTRACT

Provided are an optical device including a multilayer reflector having a layer whose optical thickness is not λ/4, and a vertical cavity surface emitting laser using the optical device. A resonance frequency shift or a reduction in reflectivity which is caused by a deviation from an optical thickness of λ/4 can be suppressed to improve characteristics and yield. The optical device for generating light of a wavelength λ includes a reflector and an active layer. The reflector is a semiconductor multilayer reflector including a first layer and a second layer which are alternatively laminated and have different refractive indices. The first layer has an optical thickness smaller than λ/4. The second layer has an optical thickness larger than λ/4. The interface between the first layer and the second layer is located at neither a node nor an antinode of an optical intensity distribution within the reflector.

This is a division of U.S. patent application Ser. No. 11/782,221, filedJul. 24, 2007 now U.S. Pat. No. 7,756,187.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device including amultilayer reflector.

2. Description of the Related Art

(Vertical Cavity Surface Emitting Laser)

A vertical cavity surface emitting laser (VCSEL) which is an opticaldevice can emit light in a direction perpendicular to a semiconductorsubstrate, so a two-dimensional array can be easily formed. Whenparallel processing of multibeam emitted from the two-dimensional arrayis performed, higher density and higher speed can be obtained, sovarious industrial applications are expected. For example, when avertical cavity surface emitting laser array is used as an exposuresource of an electrophotographic printer, the printing speed can beincreased by parallel processing of a printing process using themultibeam.

A vertical cavity surface emitting laser which is currently in practicaluse is a device for mainly generating laser light of an infrared region(0.75 μm to 0.85 μm). A beam spot can be further reduced as anoscillation wavelength is shortened from the infrared region to a redregion, a blue region, and an ultraviolet region, so higher resolutioncan be obtained. Therefore, the practical use of a vertical cavitysurface emitting laser in the regions from red to ultraviolet isrequired.

There is a great effect attained by a combination of the increase inresolution which is obtained by the shortened wavelength and theparallel processing using the multibeam. Its contribution to variousfields including applications for a printer is expected. When a verticalcavity surface emitting laser capable of oscillating in a band of from1.3 μm to 1.5 μm in which dispersion or absorption in an optical fiberis small can be in practical use, long-distance large-capacitycommunications can be performed using an arrayed fiber and an arrayedoptical source.

(Multilayer Reflector)

The feature of the vertical cavity surface emitting laser is to includea cavity provided in a direction perpendicular to an in-plane directionof a substrate. In order to realize a surface-emitting laser which cancontinuously operate at a room temperature, a reflector whosereflectivity is 99% or more is necessary.

An example of such a reflector to be used includes a multilayerreflector in which two materials whose refractive indices are differentfrom each other are alternately laminated plural times by an opticalthickness of λ/4. Here, λ denotes a wavelength of light emitted from anoptical device. The optical thickness is obtained by multiplying athickness of a layer by a refractive index of a material of the layer.

(Near-Infrared Vertical Cavity Surface Emitting Laser)

For a near-infrared vertical cavity surface emitting laser using a GaAssemiconductor which is already in a practical use, a semiconductormultilayer mirror in which GaAs and AlAs which have extremely highcrystallinity are combined is used. In addition, a semiconductormultilayer mirror in which AlGaAs whose Al composition is small andAlGaAs whose Al composition is large are combined for constituent layersis used.

However, a long-wavelength (1.3 μm to 1.5 μm) laser for communicationand a red (0.62 μm to 0.7 μm) laser have a problem in that their thermalcharacteristics are undesirable or high-power output is difficult to beachieved.

That is, in an active layer for generating light of the communicationwavelength region or the red region, there is no clad layer materialcapable of sufficiently confining electrons in the active layer at ahigh temperature ranging from 60° C. to 80° C. Therefore, with anincrease in temperature, a large number of electrons are overflowingfrom the active layer, so the thermal characteristics deteriorate andthe high-power output is difficult to perform.

In the vertical cavity surface emitting laser, heat generated by theactive layer is confined to the vicinities of the active layer by thesemiconductor multilayer reflector whose thermal resistance is high.

Therefore, unfortunately, the vertical cavity surface emitting lasersusing the material for generating the light of the above-mentionedwavelength are devices whose temperature characteristics areundesirable.

To be specific, a normal multilayer reflector of the long-wavelengthvertical cavity surface emitting laser has a structure in which anInGaAsP layer (high refractive index layer) whose optical thickness isλ/4 and an InP layer (low refractive index layer) whose opticalthickness is λ/4 are alternately laminated as a large number of pairs.In this case, the thermal resistance of the InGaAsP layer used as thehigh refractive index layer is approximately 20 times larger than thethermal resistance of the InP layer used as the low refractive indexlayer.

In the circumstances, in 47th Seiken Symposium Preprints, pp. 80-81,March 2006 (Precision and Intelligence Laboratory, Tokyo Institute ofTechnology), a multilayer reflector in which the optical thickness of amultilayer reflector constituent layer is not set to λ/4 is discussed.

To be specific, with respect to the multilayer reflector of thelong-wavelength vertical cavity surface emitting laser, in 47th SeikenSymposium Preprints, pp. 80-81, March 2006 (Precision and IntelligenceLaboratory, Tokyo Institute of Technology), a multilayer reflector inwhich the optical thickness of an InP layer whose thermal resistance issmall is set to a value larger than λ/4 and the optical thickness of anInGaAsP layer whose thermal resistance is large is set to a valuesmaller than λ/4 is disclosed in order to reduce the thermal resistance.

The total layer thickness (one pair) of the high refractive index layerand the low refractive index layer which constitute the multilayerreflector is fixed to be an optical thickness of λ/2. Therefore, it isconsidered that a heat dissipation effect can be improved and thus it ispossible to provide the multilayer reflector in which an increase indevice temperature can be prevented.

(Ultraviolet/Blue Vertical Cavity Surface Emitting Laser)

A GaN semiconductor material is used for a vertical cavity surfaceemitting laser in an ultraviolet/blue region (300 μm to 500 μm). For amultilayer reflector, for example, a pair including a GaN material andan AlN material with a relatively large refractive index differencetherebetween is selected.

However, the multilayer reflector made of both of these materials has alarge lattice mismatch. When several ten pairs are grown by an opticalthickness of λ/4, it is more likely to introduce lattice strains intothe multilayer by the lattice mismatch. As a result, a crack occurs, soit is difficult to form such a multilayer reflector as to achieve areflectance of 99% or more.

Therefore, also in Japanese Patent Application Laid-Open No.2003-107241, a multilayer reflector in which the optical thickness ofconstituent layers of the multilayer reflector is not set to λ/4 isdiscussed.

To be specific, a multilayer reflector is disclosed in which the opticalthickness of a GaN layer whose thermal expansion coefficient differencewith respect to a substrate is small is set to a value larger than λ/4and the optical thickness of a Al_(0.6)Ga_(0.4)N layer whose thermalexpansion coefficient difference with respect to the substrate is largeis set to a value smaller than λ/4.

The total optical thickness (of one pair) of the high refractive indexlayer and the low refractive index layer which constitute the multilayerreflector is fixed to be an optical thickness of λ/2. Therefore, it isconsidered that the multilayer reflector with few cracks can beprovided.

In 47th Seiken Symposium Preprints, pp. 80-81, March 2006 (Precision andIntelligence Laboratory, Tokyo Institute of Technology) and JapanesePatent Application Laid-Open No. 2003-107241, the multilayer reflectorincluding a layer whose optical thickness is not λ/4 is described.However, a design guideline for actually incorporating the multilayerreflector into a cavity is not described therein.

The inventor of the present invention arranged the layers whose opticalthickness is not λ/4 as described in 47th Seiken Symposium Preprints,pp. 80-81, March 2006 (Precision and Intelligence Laboratory, TokyoInstitute of Technology) and studied in view of a cavity structure. As aresult, it was found that the arrangement causes a deviation of a designvalue from a resonance wavelength and a reduction in reflectivity.

That is, when the arrangement of the high refractive index layer and thelow refractive index layer with respect to the internal opticalintensity distribution is not taken into account, it is likely to causea reduction in yield due to the deviation of the resonance frequency ordeterioration of device characteristics due to a reduction inreflectivity. As a result, although the intended use of the layer whoseoptical thickness is not λ/4 is made for improving the characteristics,a problem that the effect of such use cannot be obtained occurs.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-mentioned problemand an object of the present invention is to provide an optical deviceincluding a reflector having a layer whose optical thickness is not λ/4,for which the resonance wavelength is made closer to a design value anda reduction in reflectivity is suppressed.

According to the present invention, there is provided an optical devicefor generating light of a wavelength λ, including a reflector and anactive layer, the reflector being a semiconductor multilayer reflectorincluding a first layer and a second layer which are alternativelylaminated and have refractive indices different from each other, whereinthe first layer has an optical thickness smaller than λ/4 and the secondlayer has an optical thickness larger than λ/4, and an interface betweenthe first layer and the second layer is located at a position other thana node and an antinode of an optical intensity distribution within thereflector.

According to the present invention, with respect to the optical deviceincluding the reflector having the layer whose optical thickness is notλ/4, the resonance wavelength can be made closer to the design value andthe reduction in reflectivity can be suppressed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view illustrating a structure of avertical cavity surface emitting laser according to an embodiment of thepresent invention.

FIGS. 2A, 2B, 2C, and 2D are enlarged schematic views illustratingresonator structures of the vertical cavity surface emitting laser ofFIG. 1.

FIG. 3 is a schematic cross sectional view illustrating a structure of avertical cavity surface emitting laser according to Example 1.

FIGS. 4A, 4B, 4C and 4D are enlarged schematic views illustratingresonator structures each including an AlAs/Al_(0.5)Ga_(0.5)Asmultilayer reflector and a Ga_(0.5)In_(0.5)P quantum well active layeras illustrated in FIGS. 2A to 2D.

FIG. 5 illustrates a comparison between center wavelengths andreflectances in cases where only the multilayer reflector as illustratedin FIGS. 6A to 6D is taken into account.

FIGS. 6A, 6B, 6C and 6D are enlarged schematic views illustratingstructures in which only the AlAs/Al_(0.5)Ga_(0.5)As multilayerreflector is taken into account.

FIG. 7 illustrates a comparison between center wavelengths andreflectances in cases where a resonator including the multilayerreflector and the active layer illustrated in FIGS. 4A to 4D is takeninto account.

FIGS. 8A and 8B are schematic explanatory views illustrating a specificstructure (FIG. 8B) of Example 1 in comparison with a conventionalexample (FIG. 8A).

FIG. 9 is a schematic cross sectional view illustrating a structure of avertical cavity surface emitting laser according to Example 2.

FIGS. 10A, 10B, 100 and 10D are enlarged schematic views illustratingresonator structures each including an AlN/GaN multilayer reflector andan InGaN quantum well active layer as illustrated in FIG. 9.

FIG. 11 illustrates a comparison between center wavelengths andreflectances in cases where only the multilayer reflector as illustratedin FIGS. 12A to 12D is taken into account.

FIGS. 12A, 12B, 12C and 12D are enlarged schematic views illustratingstructures in which only the AlN/GaN multilayer reflector is taken intoaccount.

FIG. 13 illustrates a comparison between center wavelengths andreflectances in cases where a resonator including the multilayerreflector and the active layer illustrated in FIGS. 10A to 10D are takeninto account.

FIGS. 14A and 14B are schematic explanatory views illustrating aspecific structure (FIG. 14B) of Example 2 in comparison with aconventional example (FIG. 14A).

FIG. 15 is a schematic cross sectional view illustrating a structure ofa vertical cavity surface emitting laser according to Example 3.

FIG. 16 is an enlarged schematic view illustrating a resonator structureincluding an AlN/GaN multilayer reflector and an InGaN quantum wellactive layer as illustrated in FIG. 15.

FIG. 17 is a schematic cross sectional view illustrating a structure ofa vertical cavity surface emitting laser according to Example 4.

FIG. 18 is an enlarged schematic view illustrating a resonator structureincluding an AlN/GaN multilayer reflector and an InGaN quantum wellactive layer as illustrated in FIG. 17.

FIG. 19 is a schematic explanatory view illustrating anelectrophotographic apparatus using an optical device according to thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described with reference toFIG. 1 and FIGS. 2A to 2D.

FIG. 1 is a schematic cross sectional view illustrating a structure ofan optical device according to an embodiment of the present invention.Here, an example of a vertical cavity surface emitting laser will bedescribed. A multilayer reflector 102, a spacer layer 103, an activelayer 104, a spacer layer 105, and a multilayer reflector 106 areprovided on a substrate 101.

FIGS. 2A to 2D are enlarged schematic views illustrating the multilayerreflector 102, the spacer layer 103, and the active layer 104 shown inFIG. 1. In the case of the multilayer reflector 102, only three pairslocated close to the active layer 104 are illustrated for the sake ofconvenience. FIGS. 2A and 2B illustrate reference examples, FIG. 2Cillustrates an embodiment of the present invention, and FIG. 2Dillustrates a conventional example.

A. Structure of Conventional Example

In the conventional example shown in FIG. 2D, a multilayer reflector inwhich the optical thicknesses of both layers are λ/4 is constructed withconventional design values. As illustrated in FIG. 2D, the multilayerreflector 102 includes a layer 110 and a layer 120 which are alternatelylaminated. The layer 120 has a refractive index different from therefractive index of the layer 110. It is only required that the layers110 and 120 have different refractive indexes from each other, so therefractive index of the layer 110 may be higher than or lower than therefractive index of the layer 120. FIG. 2D illustrates the case wherethe refractive index of the layer 120 is lower than the refractive indexof the layer 110.

B. Structures of Reference Examples

In the reference examples of FIGS. 2A and 2B, a multilayer reflector inwhich the optical thicknesses of both layers deviate from λ/4 isconstructed. As illustrated in FIGS. 2A and 2B, the multilayer reflectorincludes a first layer 130 and a second layer 140 which are alternatelylaminated. It is only required that the first layer 130 and the secondlayer 140 have different refractive indexes, so the refractive index ofthe first layer 130 may be higher than or lower than the refractiveindex of the second layer 140.

Unlike the structure of the conventional example, in the referenceexamples, the optical thickness of the first layer 130 is smaller thanλ/4 and the optical thickness of the second layer 140 is larger thanλ/4.

The optical thickness is obtained by multiplying a thickness of a layerby a refractive index of a material of the layer. For example, assumethat the film thickness of the first layer 130 is expressed by d_(A),the refractive index thereof is expressed by n_(A), the film thicknessof the second layer 140 is expressed by d_(B), and the refractive indexthereof is expressed by n_(B). Then, the optical thickness of the firstlayer 130 is expressed by d_(A)n_(A) and the optical thickness of thesecond layer 140 is expressed by d_(B)n_(B).

The optical thickness of the first layer 130 and the optical thicknessof the second layer 140 can be set as appropriate. For example, theoptical thickness of the first layer 130 can be set to λ/8 or less andthe optical thickness of the second layer 140 can be set to 3λ/8 ormore. Alternatively, the optical thickness of the first layer 130 can beset to λ/16 or less and the optical thickness of the second layer 140can be set to 7λ/16 or more.

In order to obtain a resonance wavelength as designed, it is desirableto set the sum of the optical thickness of the first layer 130 and theoptical thickness of the second layer 140 to λ/2.

An internal optical intensity distribution 210 of the multilayerreflector are illustrated on the right side of FIG. 2D. The internaloptical intensity distribution 210 includes an antinode 220 and a node230.

As illustrated in FIG. 2A, one interface located between the first layer130 and the second layer 140 corresponds to the antinode 220 of theoptical intensity distribution. As illustrated in FIG. 2B, one interfacelocated between the first layer 130 and the second layer 140 correspondsto the node 230 of the optical intensity distribution.

C. Structure of this Embodiment

As in the reference example, in this embodiment, it is only requiredthat the first layer 130 and the second layer 140 have differentrefractive indices, so the refractive index of the first layer 130 maybe higher than or lower than the refractive index of the second layer140. The optical thickness of the first layer 130 is smaller than λ/4and the optical thickness of the second layer 140 is larger than λ/4.

However, as illustrated in FIG. 2C, in the embodiment of the presentinvention, the interfaces between the first layer 130 and the secondlayer 140 correspond to neither the antinode 220 nor the node 230 of theoptical intensity distribution.

Starting from the conventional example of FIG. 2D, when the thicknessesof both layers which constitute a multilayer reflector are changed, theinterface between the layers is normally arranged so as to correspond toan antinode of the optical intensity distribution or a node thereof asillustrated in FIG. 2A or 2B. Even in the case of 47th Seiken SymposiumPreprints, pp. 80-81, March 2006 (Precision and Intelligence Laboratory,Tokyo Institute of Technology), a method based on FIG. 2A or 2B isemployed.

However, as illustrated in FIG. 2C, the feature of the embodiment of thepresent invention is to employ the structure in which the interfacebetween the layers is arranged corresponding to neither the antinode northe node of the optical intensity distribution.

When such a structure is employed, the resonance wavelength closer tothe design value can be obtained and a reduction in reflectivity can besuppressed.

A material of the first layer 130 whose optical thickness is small and amaterial of the second layer 140 whose optical thickness is large can beselected as appropriate depending on purposes. Hereinafter, applicationexamples to a long-wavelength laser and a short-wavelength laser will bedescribed.

(Application to Long-Wavelength Laser)

As described above, a long-wavelength (1.3 μm to 1.5 μm) laser used forcommunication and a red (0.62 μm to 0.7 μm) laser have a problem in thattheir thermal characteristics are undesirable or high-power output isdifficult to obtain. The vertical cavity surface emitting laser has aproblem in that heat generated by an active layer is confined to thevicinities of the active layer by a semiconductor multilayer reflectorwhose thermal resistance is high.

To be specific, when an Al_(0.5)Ga_(0.5)As layer and an AlAs layer beinga binary system material are used respectively as a high refractiveindex layer and a low refractive index layer of a multilayer reflectorfor a red vertical cavity surface emitting laser, the thermal resistanceof the Al_(0.5)Ga_(0.5)As layer is eight times or more larger than thethermal resistance of the AlAs layer.

Therefore, as will be described in Example 1, in the case of themultilayer reflector of the red vertical cavity surface emitting laser,the optical thickness of the AlGaAs layer whose thermal resistance islarge can be set smaller than λ/4 (first layer 130) and the opticalthickness of the AlAs layer whose thermal resistance is small can be setlarger than λ/4 (second layer 140).

In the case of the multilayer reflector of the long-wavelength verticalcavity surface emitting laser, the optical thickness of the InGaAsPlayer whose thermal resistance is large can be set smaller than λ/4(first layer 130) and the optical thickness of the InP layer whosethermal resistance is small can be set larger than λ/4 (second layer140).

(Application to Short-Wavelength Laser)

A GaN semiconductor material is used for a vertical cavity surfaceemitting laser in an ultraviolet/blue region (300 μm to 500 μm). Thisshort-wavelength laser has a problem in that there is no suitablematerial capable of making a lattice match with a layer serving as abase in epitaxial growth (for example, substrate) while employing alarge refractive index difference between the low refractive index layerand the high refractive index layer.

Therefore, the optical thickness of a layer whose lattice mismatch issmall can be set larger than λ/4 and the optical thickness of a layerwhose lattice mismatch is large can be set smaller than λ/4. Thus, it ispossible to provide a multilayer reflector in which high reflectance canbe realized while the risk of causing a crack is suppressed.

For example, when GaN, GaN, and AlN are used for the substrate, the highrefractive index layer, and the low refractive index layer,respectively, the lattice mismatch between GaN of the substrate and AlNof the low refractive index layer is large.

Therefore, as will be described in Example 2, in the case of themultilayer reflector of the short-wavelength vertical cavity surfaceemitting laser, the optical thickness of the AlN layer whose latticemismatch with the substrate is large but whose refractive indexdifference with respect to the high refractive index layer is increasedcan be set smaller than λ/4. The optical thickness of the GaN layerwhich has a small lattice mismatch with the substrate and is used as thehigh refractive index layer can be set larger than λ/4.

In view of thermal expansion coefficients, the optical thickness of theAlGaN layer whose thermal expansion coefficient difference with respectto the substrate is large can be set smaller than λ/4 and the opticalthickness of the GaN layer whose thermal expansion coefficientdifference with respect to the substrate is small can be set larger thanλ/4.

Another Embodiment

The number of pairs of the first layer 130 and the second layer 140which are laminated is desirably two or more. When the number oflaminated pairs increases, the reflectance of the multilayer reflectorbecomes higher. When the refractive index difference between the firstlayer 130 and the second layer 140 increases, the reflectivity of themultilayer reflector becomes higher.

The optical device according to the present invention can be used forvarious optical devices including not only the vertical cavity surfaceemitting laser but also a light-emitting diode and an optical functiondevice. For example, when the number of pairs of the multilayerreflector in the present invention reduces, the optical device accordingto the present invention can be used as a light-emitting diode.

According to the present invention, an upper and lower multilayerreflectors are not necessarily provided. The present invention alsoincludes an optical device having at least one single multilayerreflector.

The optical device according to the present invention can be suitablyused as a light source of an electrophotographic recording process imageforming apparatus.

EXAMPLE Example 1 AlAs/AlGaAs

In Example 1, a vertical cavity surface emitting laser including amultilayer reflector which is used as a multilayer reflector for a redvertical cavity surface emitting laser and is made of an AlAs materialand an AlGaAs material will be described. This multilayer reflector isconstructed to reduce a thermal resistance.

FIG. 3 is a schematic cross sectional view illustrating a structure ofthe vertical cavity surface emitting laser according to Example 1.

In this example, the vertical cavity surface emitting laser includes aGaAs substrate 400, an n-type AlAs/Al_(0.5)Ga_(0.5)As multilayerreflector 410, an n-type Al_(0.5)Ga_(0.15)In_(0.5)P spacer layer 405, aGa_(0.5)In_(0.5)P/Al_(0.25)Ga_(0.25)In_(0.5)P quantum well active layer404, a P-type Al_(0.35)Ga_(0.15)In_(0.5)P spacer layer 409, and a p-typeAl_(0.9)Ga_(0.1)As/Al_(0.5)Ga_(0.5)As multilayer reflector 420.

The Ga_(0.5)In_(0.5)P/Al_(0.25)Ga_(0.25)In_(0.5)P quantum well activelayer 404 used here includes, for example, four Ga_(0.5)In_(0.5)P welllayers and has a light emission wavelength λ of 650 nm to 690 nm.

The thickness of the n-type Al_(0.35)Ga_(0.15)In_(0.5)P spacer layer 405and the thickness of the P-type Al_(0.35)Ga_(0.15)In_(0.5)P spacer layer409 are adjusted to use an optical thickness of one wavelength as acavity length. If necessary, the light emission wavelength, the numberof wells, or the cavity length can be adjusted.

The p-type Al_(0.9)Ga_(0.1)As/Al_(0.5)Ga_(0.5)As multilayer reflector420 is formed such that each optical thickness is λ/4 as in aconventional design. In order to reduce electrical resistance, acomposition gradient layer of approximately 20 nm may be providedbetween the Al_(0.9)Ga_(0.1)As layer and the Al_(0.5)Ga_(0.5)As layer.Even in this case, the multilayer reflector 420 is formed such that theoptical thickness of the layers including the composition gradient layeris λ/4.

In contrast to this, in order to reduce thermal resistance of the n-typeAlAs/Al_(0.5)Ga_(0.5)As multilayer reflector 410, the opticalthicknesses of the two constituent layers thereof are not λ/4. Theoptical thickness of the AlAs layer whose thermal resistance is small isset larger than λ/4 and the optical thickness of the Al_(0.5)Ga_(0.5)Aslayer whose thermal resistance is large is set smaller than λ/4.

In this example, the optical thickness of the AlAs layer is set to 3λ/8and the optical thickness of the Al_(0.5)Ga_(0.5)As layer is set to λ/8.Note that the sum of the layer thickness of the layers is maintained atan optical thickness of λ/2 in order to prevent the resonance wavelengthfrom shifting.

(Cavity Structure)

Hereinafter, a method of incorporating the multilayer reflector in whichfilm thicknesses are modulated into a cavity structure will bedescribed.

FIGS. 4A to 4D are schematic enlarged views illustrating theAlAs/Al_(0.5)Ga_(0.5)As multilayer reflector 410, theGa_(0.5)In_(0.5)P/Al_(0.25)Ga_(0.25)In_(0.5)P quantum well active layer404, and the Al_(0.35)Ga_(0.15)In_(0.5)P spacer layer 405 shown in FIG.3.

FIGS. 4A to 4D illustrate an internal optical intensity distribution210, an antinode 220 of the internal optical intensity distribution 210,a node 230 of the internal optical intensity distribution 210, an AlAslayer 406, and an Al_(0.5)Ga_(0.5)As layer 407. For simplicity, onlythree pairs in the multilayer reflector are illustrated in FIGS. 4A to4D. The AlAs layer 406 is a low refractive index layer and theAl_(0.5)Ga_(0.5)As layer 407 is a high refractive index layer.

In this example, the optical thickness of the Al_(0.5)Ga_(0.5)As layer407 whose thermal resistance is large is set to λ/8 to obtain a thinfilm and the optical thickness of the AlAs layer 406 whose thermalresistance is small is set to 3λ/8 to obtain a thick film.

Hereinafter, the following respective cases of FIGS. 4A to 4D will bediscussed based on the arrangement of the thickened AlAs layer 406 andthe thinned Al_(0.5)Ga_(0.5)As layer 407.

FIGS. 4A and 4B illustrate reference examples. FIG. 4A indicates thecase where the interface between the AlAs layer 406 and theAl_(0.5)Ga_(0.5)As layer 407 is arranged so as to correspond to theantinode 220 of the optical intensity distribution and FIG. 4B indicatesthe case where the interface is arranged so as to correspond to the node230 of the optical intensity distribution.

FIG. 4C illustrates Example 1 and the interface is located correspondingto neither an antinode nor a node of the optical intensity distribution.FIG. 4C indicates a more desirable example in which the center of thethickened AlAs layer 406 or the center of the thinned Al_(0.5)Ga_(0.5)Aslayer 407 is located corresponding to just a midpoint between anantinode and a node, which are adjacent to each other, of the opticalintensity distribution.

FIG. 4D illustrates a conventional example. For comparison with Example1, the multilayer reflector is constructed with the optical thickness ofλ/4 based on conventional design values. Note that, in this example, anAl_(0.9)Ga_(0.1)As layer 408 is used as a low refractive index layersinstead of the AlAs layer 406.

(Reflectivity and Resonance Wavelength)

FIG. 5 illustrates a calculation result of the reflectivity and thecenter wavelength of the reflection stop band for each of theabove-mentioned multilayer reflectors. They are designed such that thecenter wavelength of the reflection stop band becomes 670 nm. Thereflectivity in a direction indicated by each arrow is calculated basedon the assumption that there is only a lower reflector as illustrated inFIGS. 6A to 6D. Assume that a conventional multilayer reflector servingas a comparison example, which is designed with the optical thickness ofλ/4, has 70 pairs of layers. In order to obtain a reflectancesubstantially equal to the reflectance of the conventional multilayerreflector, pairs of layers are further added to provide 76 pairs oflayers in total in each of the cases of FIGS. 6A, 6B, and 6C.

As is apparent from the table of FIG. 5, when only the multilayerreflector is taken into account, substantially the same reflectance(99.997%) and substantially the same center wavelength (670 nm) areobtained in FIGS. 6A, 6B, and 6C irrespective of the arrangement of thethin film layers or the thick film layers. That is, even when theoptical thickness deviates from λ/4, the positions of the thinned orthickened layers in the calculation performed in view of only thereflector do not cause a problem.

Next, FIG. 7 illustrates a calculation result of the resonancewavelength and the reflectance at the wavelength in the case where thecavity structure is incorporated, that is, in the case of each of FIGS.4A to 4D. In the case of the conventional multilayer reflector designedwith the optical thickness of λ/4, that is, in the case of FIG. 4D, thesame result (670 nm, 99.997% or more) is obtained from both thecalculation with respect to only the multilayer reflector and thecalculation with respect to the cavity in which the multiple reflectoris incorporated.

In contrast to this, it is apparent that the resonance wavelength in thecase of FIG. 4A is shifted to longer wavelengths by approximately 8 nmand the resonance wavelength in the case of FIG. 4B is shifted toshorter wavelengths by approximately 8 nm. The reflectance is reduced tothe order of 99.98%.

On the other hand, in the case of FIG. 4C, although the multilayerreflector is comprised of the layers whose optical thicknesses are notλ/4, the calculation result with respect to only the multilayerreflector and that with respect to the resonator in which the multiplereflector is incorporated are substantially equal to each other as inthe conventional case of FIG. 4D.

That is, in the case of the multilayer reflector using layers whoseoptical thicknesses deviate from λ/4 as described above, unless not onlythe multilayer reflector but also the cavity in which the multilayerreflector is incorporated is taken into account, a deviation of theresonance wavelength or a reduction in reflectivity unexpectedly occurs.In the case where the structure in this example is employed, even whenthe optical thickness deviates from λ/4, the characteristics of the mainbody can be brought out without being damaged.

(Design Guide)

Next, a specific structure of FIG. 4C will be described.

FIG. 8B illustrates the structure of FIG. 4C in this example and FIG. 8Aillustrates the structure of FIG. 4D in the conventional example.

In FIGS. 8A and 8B, λ denotes an oscillation wavelength of laser lightand n₁ denotes an average refractive index of a medium located betweenan active layer and at least one of a first mirror and a second mirror.In addition, n₂ denotes a refractive index of a thickened layer whoseoptical thickness is larger than λ/4 (AlAs layer) and n₃ denotes arefractive index of a thinned layer whose optical thickness is smallerthan λ/4 (Al_(0.5)Ga_(0.5)As layer). The optical thickness of thethinned Al_(0.5)Ga_(0.5)As layer becomes smaller than λ/4 (λ/4n₃) and isexpressed here by x.

In this example, the AlAs/Al_(0.5)Ga_(0.5)As multilayer reflector whichis one of the multiple reflectors in the vertical cavity surfaceemitting laser is arranged as illustrated in FIG. 8B. That is, it is soarranged that the center of the Al_(0.5)Ga_(0.5)As layer having athickness of x is located at a distance, from the center of the activelayer, of an integral multiple of λ/2n₁, λ/4n₂, and a distance definedby the following expression

$\frac{\lambda}{8n_{2}} + {\frac{n_{2} - n_{3}}{2n_{2}}x}$and the AlAs/Al_(0.5)Ga_(0.5)As layers are repeatedly provided at aninterval defined by the following expression:

$\frac{\lambda}{2n_{2}} + {\frac{n_{2} - n_{3}}{n_{2}} \cdot x}$

An example of the structure expressed by this expression includes astructure in which the layer thickness of the AlAs layer whose thermalconductivity is high is set to 3λ/8n₂ and the layer thickness of theAl_(0.5)Ga_(0.5)As layer whose thermal conductivity is low is set toλ/8n₃.

Example 2 AlN/GaN

Example 2 will be described. In Example 2, a vertical cavity surfaceemitting laser which includes a multilayer reflector having an AlN layerand a GaN layer and is used for emitting ultraviolet/blue light will bedescribed. The reflector is constructed to improve a reflectance in viewof refractive index difference and lattice mismatch.

FIG. 9 is a schematic cross sectional view illustrating a structure ofthe vertical cavity surface emitting laser according to Example 2.

In this example, the vertical cavity surface emitting laser includes aGaN substrate 701, an AlN/GaN multilayer reflector 702, a GaN spacerlayer 703, an InGaN quantum well active layer 704, and an SiO₂/TiO₂multilayer reflector 705.

The InGaN quantum well active layer 704 used here includes, for example,four InGaN well layers and has a light emission wavelength of 390 nm to410 nm. The optical thicknesses of the two GaN spacers 103 are adjustedto obtain a cavity length corresponding to 2 wavelengths. If necessary,the light emission wavelength, the number of wells, or the resonatorlength can be adjusted.

According to the conventional design method, the optical thickness ofeach of the AlN layer and the GaN layer which constitute the multiplereflector 702 is λ/4.

In contrast to this, according to this example, the GaN substrate isused, so the optical thickness of the AlN layer having a larger latticemismatch is set to λ/8, the optical thickness of the GaN layer havinglittle strain is set to 3λ/8, and 24 pairs of the AlN layer and the GaNlayer are laminated.

The SiO₂/TiO₂ multilayer reflector 705 includes eight pairs of layerswhich are laminated with the conventional layer thickness correspondingto the optical thickness of λ/4. Doping and electrode formation whichare necessary for current injection are omitted here because of havingno direct relation with the present invention. However, when doping andelectrode formation are suitably performed, it is possible to provide astructure in which a current can be injected.

(Cavity Structure)

FIGS. 10A to 10D are schematic enlarged views illustrating a cavitystructure including the AlN/GaN multilayer reflector 702 and the InGaNquantum well active layer 704 shown in FIG. 9.

FIGS. 10A to 10D illustrate an internal optical intensity distribution210, an antinode 220 of the internal optical intensity distribution 210,a node 230 of the internal optical intensity distribution 210, an InGaN704 quantum well, an AlN layer 805, and a GaN layer 806. Forsimplification, only three pairs in the multilayer reflector areillustrated in FIGS. 10A to 10D. The AlN layer 805 is a low refractiveindex layer and the GaN layer 806 is a high refractive index layer.

There is a large difference between a lattice constant of the substratewhich is generally used for a group III nitride semiconductor laser or alattice constant of a thickest group III nitride semiconductor layerserving as a base in epitaxial growth and a lattice constant of the AlNlayer.

On the other hand, the AlN layer has a low refractive index, so arefractive index difference with respect to a high refractive indexlayer such as the GaN layer is large, thereby improving thereflectivity. Therefore, in this example, the optical thickness of theAlN layer whose lattice mismatch is large is set to λ/8 and the opticalthickness of the GaN layer whose lattice mismatch is little is set to3λ/8.

Hereinafter, the following respective cases of FIGS. 10A to 10D will bediscussed on the thinned AlN layer based on the arrangement thereof.

FIGS. 10A and 10B illustrate reference examples. FIG. 10A indicates thecase where one GaN/AlN interface is located corresponding to theantinode 220 of the optical intensity distribution 210 and FIG. 10Bindicates the case where one interface is located corresponding to thenode 230 of the optical intensity distribution 210.

FIG. 10C illustrates Example 2 and the interface is locatedcorresponding to neither the antinode nor the node of the opticalintensity distribution. FIG. 10C indicates a more desirable example inwhich the center of the thinned AlN layer 805 is located correspondingto just a midpoint between an antinode and a node, which are adjacent toeach other, of the optical intensity distribution.

FIG. 10D illustrates a conventional example. For comparison with Example2, the multilayer reflector is constructed with the optical thickness ofλ/4 based on conventional design values. Note that, in this example, anAl_(0.5)Ga_(0.5)N layer 807 instead of the AlN layer 805 is used as alow refractive index layer.

In order to facilitate a relationship among the constituent layerinterfaces and the antinodes/nodes of the internal optical intensitydistribution, the intensity distribution is illustrated on the rightside.

(Reflectivity and Resonance Wavelength)

FIG. 11 illustrates a calculation result of the reflectivity and thecenter wavelength of a reflection stop band in each of theabove-mentioned multilayer reflectors. They are designed such that thecenter wavelength of the reflection stop band becomes 400 nm. Thereflectance in a direction indicated by each arrow is calculated basedon the assumption that there is only a lower reflector as illustrated inFIGS. 12A to 12D. In each of the cases of FIGS. 12A to 12D, 24 pairs oflayers are provided.

As is apparent from the table of FIG. 11, when only the multilayerreflector is taken into account, substantially the same reflectance(99.5%) and substantially the same center wavelength (400 nm) areobtained in FIGS. 12A, 12B, and 12C irrespective of the arrangement ofthe thin film layers or the thick film layers. That is, even when theoptical thickness deviates from λ/4, the positions of the thinned layersin the calculation performed in view of only the reflector do not causea problem.

Next, FIG. 13 illustrates a calculation result of the resonancewavelength and the reflectance in the case where the cavity isincorporated, that is, in the case of each of FIGS. 10A to 10D. In thecase of the conventional multilayer reflector designed with the opticalthickness of λ/4, that is, in the case of FIG. 10D, the same result (400nm, 99.12%) is obtained from both the calculation with respect to onlythe multilayer reflector and the calculation with respect to the cavityin which the multiple reflector is incorporated.

In contrast to this, in a case of FIGS. 10A and 10B, it is apparent thatthe resonance wavelength in the case of FIG. 10A is shifted to shorterwavelengths and the resonance wavelength in the case of FIG. 10B isshifted to longer wavelengths by approximately 9 nm to 10 nm,respectively. The reflectivity is reduced to the order of 98%.

On the other hand, in the case of FIG. 10C, although the multilayerreflector is comprised of the layers whose optical thicknesses are notλ/4, a calculation result with respect to only the multilayer reflectorand that with respect to the cavity in which the multiple reflector isincorporated are substantially equal to each other as in theconventional case of FIG. 10D, and reduction in a reflectivity anddeviation of the resonance wavelength are not observed.

That is, in the case of the multilayer reflector using layers whoseoptical thicknesses deviate from λ/4 as described above, unless not onlythe multilayer reflector but also the cavity in which the multilayerreflector is incorporated is taken into account, a deviation of theresonance wavelength or a reduction in reflectivity unexpectedly occurs.

In the case where the structure in this example is employed, even whenthe optical thickness deviates from λ/4, the characteristics of the mainbody can be brought out without being damaged.

As described above, according to this example, a group III nitridesemiconductor multilayer reflector whose reflectivity is high and crackis small in number can be easily manufactured, and the vertical cavitysurface emitting laser using the multilayer reflector can be realized.

(Design Guide)

Next, a specific structure of FIG. 10C in this example will be furtherdescribed.

FIG. 14B illustrates the structure of FIG. 100 in this example and FIG.14A illustrates the structure of FIG. 10D in the conventional example.

In FIGS. 14A and 14B, λ denotes an oscillation wavelength of laser lightand n₁ denotes an average refractive index of a medium located betweenan active layer and at least one of the first mirror and the secondmirror.

In addition, n₂ denotes a refractive index of a group III nitridesemiconductor layer whose lattice mismatch with respect to the substrateis larger (AlN layer) and n₃ denotes a refractive index of a group IIInitride semiconductor layer whose lattice mismatch with respect to thesubstrate is smaller (GaN layer). The optical thickness of the AlN layerwhose lattice mismatch is larger is smaller than λ/4(λ/4n₂) and isexpressed here by x.

In this example, the AlN/GaN multilayer reflector which is one of themultiple reflectors in the vertical cavity surface emitting laser isarranged as illustrated in FIG. 14. That is, it is so arranged that thecenter of the AlN layer having a thickness of x which is smaller thanλ/4n₂ is located at a distance, from the center of the active layer, ofan integral multiple of λ/2n₁ and a distance defined by the followingexpression

$\frac{\lambda}{8n_{3}} + {\frac{n_{3} - n_{2}}{2n_{3}}x}$and the AlN/GaN layers are repeatedly provided at an interval defined bythe following expression:

$\frac{\lambda}{2n_{3}} + {\frac{n_{3} - n_{2}}{n_{3}}x}$

An example of the structure expressed by this expression includes astructure in which a semiconductor layer with larger lattice mismatch isused as the AlN layer with a layer thickness of λ/8n₂ and asemiconductor layer with smaller lattice mismatch is used as the GaNlayer with a layer thickness of 3λ/8n₃.

Example 3 AlN/GaN

Example 3 will be described. In Example 3, a vertical cavity surfaceemitting laser will be described in which a substrate (sapphire)different from the substrate in Example 2 is used and multilayerreflectors are formed above and below an active layer using differentfilm thicknesses.

FIG. 15 is a schematic cross sectional view illustrating a structure ofthe vertical cavity surface emitting laser according to this example.

In this example, the vertical cavity surface emitting laser includes asapphire substrate 1501, a GaN thick film 1510, an AlN/GaN multilayerreflector 1502, a GaN spacer layer 1503, an InGaN quantum well activelayer 1504, and an AlN/GaN multilayer reflector 1505.

For example, the InGaN multiple quantum well active layer used here isthe same as in Example 2. Each of the multilayer reflectors 1502 and1505 includes an AlN constituent layer and a GaN constituent layer. Inthis case, the growth of the constituent layers follows the epitaxialgrowth of the GaN thick film on the sapphire substrate.

Therefore, the optical thickness of the AlN layer having a largerlattice mismatch is set to λ/16 to obtain a thin film, the opticalthickness of the GaN layer having little strain is set to 7λ/16 toobtain a thick film, and 52 pairs of the AlN layer and the GaN layer arelaminated.

Doping and electrode formation which are necessary for current injectionare omitted here because of having no direct relation with the presentinvention. However, when doping and electrode formation are suitablyperformed, it is possible to provide a structure in which a current canbe injected.

FIG. 16 is a schematic enlarged view illustrating a cavity structureincluding the AlN/GaN multilayer reflector 1502 and the InGaN quantumwell active layer 1504 as illustrated in FIG. 15.

For simplification, only three pairs included in the multilayerreflector are illustrated in FIG. 16. As in Example 2, the interfacesare located corresponding to neither the antinode 220 nor the node 230of the optical intensity distribution 210 within the multilayerreflector.

FIG. 16 indicates a more desirable example. A thinned AlN layer 1507 isarranged such that the center of the thinned AlN layer 1507 is locatedcorresponding to a midpoint between a node and an antinode, which areadjacent to each other, of the optical intensity distribution.

The thickened GaN layer 1506 is provided between the adjacent AlN layers1507. Even in this case, a reflectance of 99% or more is obtained, so ahigh reflectance which may be required for continuous oscillation atroom temperature can be realized.

The layer thickness per one multilayer reflector in this example issubstantially two times the layer thickness per one multilayer reflectorin Example 2. However, accumulated strain amounts in both the examplesare substantially equal to each other. Therefore, the multilayerreflector is resistant to cracks.

As described above, according to this example, the layer thickness to bereduced and the number of pairs can be more desirably selected inresponse to other requirements.

Example 4 AlN/GaN

Example 4 will be described. In Example 4, a vertical cavity surfaceemitting laser in which a multilayer reflector is formed using asubstrate (AlN) different from the substrate used in Example 2 or 3 willbe described. Also, this example is different from Examples 2 and 3 inthat the optical thickness of the AlN layer is thicker than the opticalthickness of the GaN layer.

FIG. 17 is a schematic cross sectional view illustrating a structure ofthe vertical cavity surface emitting laser according to this example.

In this example, the vertical cavity surface emitting laser includes anAlN substrate 1601, an AlN/GaN multilayer reflector 1602, a GaN spacerlayer 1603, an InGaN quantum well active layer 1604, and an SiO2/TiO2multilayer reflector 1605.

The InGaN quantum well active layer 1604 and the SiO2/TiO2 multilayerreflector 1605 which are used here are the same as in Example 2.

The AlN/GaN multilayer reflector 1602 includes an AlN constituent layerand a GaN constituent layer. In this case, the constituent layers areformed on the AlN substrate 1601 by epitaxial growth.

Therefore, the optical thickness of the GaN layer having a largerlattice mismatch is set to λ/8 to obtain a thin film, the opticalthickness of the AlN layer having little strain is set to 3λ/8 to obtaina thick film, and 27 pairs of the GaN layer and the AlN layer arelaminated.

Doping and electrode formation which are necessary for current injectionare omitted here because of having no direct relation with the presentinvention. However, when doping and electrode formation are suitablyperformed, it is possible to provide a structure in which a current canbe injected.

FIG. 18 is a schematic enlarged view illustrating a cavity structureincluding the AlN/GaN multilayer reflector 1602 and the InGaN quantumwell active layer 1604 as illustrated in FIG. 17.

For simplification, only three pairs in the multilayer reflector areillustrated in FIG. 18.

As in Examples 2 and 3, constituent layer interfaces are locatedcorresponding to neither the antinode 220 nor the node 230 of theoptical intensity distribution 210 within the multilayer reflector. FIG.18 indicates a more desirable example. The multilayer reflector isformed such that the center of the thinned GaN layer 1606 is locatedcorresponding to a midpoint between a node and an antinode, which areadjacent to each other, of the optical intensity distribution.

Even in this case, a reflectivity of 99% or more is obtained, so a highreflectivity which may be required for continuous oscillation at roomtemperature can be realized.

As described above, according to this embodiment, a layer to be thinnedcan be determined based on the type of a substrate to be used and thedegree of lattice mismatch caused thereby to the constituent layers ofthe multilayer reflector, to realize both a reduction of cracks and anincrease in reflectivity.

Example 5 Image Forming Apparatus

Example 5 of the present invention will be described. In Example 5, anexample in which the optical device according to the present inventionis used as a light source of an electrophotographic apparatus will bedescribed.

The electrophotographic apparatus includes a photosensitive member, acharging unit for charging the photosensitive member, a light beamemitting unit for emitting a light beam for forming an electrostaticlatent image to the charged photosensitive member, and a developing unitfor developing the electrostatic latent image formed by the emittedlight beam.

Hereinafter, an image forming process performed by theelectrophotographic apparatus will be described with reference to FIG.19.

A photosensitive member 1670 is uniformly charged by a charging unit1690. Laser light is emitted from an optical device 1640 according tothe present invention which is an exposure light source to thephotosensitive member 1670 through a polygon mirror 1650 which is anoptical path changing unit and a condensing lens 1660. When the laserlight is emitted to the photosensitive member 1670, charges are removedfrom an irradiated portion of the photosensitive member 1670 to form anelectrostatic latent image. Toner is supplied by a developing unit 1680onto the photosensitive member 1670 in which the electrostatic latentimage is formed, thereby forming a toner image. The toner image istransferred to a transferring material such as a paper.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-229100, filed Aug. 25, 2006, and No. 2007-137949, filed May 24,2007, which are hereby incorporated by reference herein in theirentirety.

1. An optical device for generating light of a wavelength λ, comprising:a multilayer reflector including a first layer having an opticalthickness of (λ/4−t) and a second layer having an optical thickness of(λ/4+t); and a cavity including a spacer layer and an active layer, andhaving an optical thickness of an integral multiple of λ/2, wherein thefirst layer has a lower refractive index than that of the second layer,wherein the layer of the multilayer that is closest to the cavity is thefirst layer, and a layer having an optical thickness of t/2 is providedbetween the multilayer reflector and the cavity.
 2. An optical deviceaccording to claim 1, wherein two or more pairs of the first layer andthe second layer are laminated.
 3. An optical device according to claim1, wherein a center of the first layer is located at a first positioncorresponding to a midpoint between a node and an antinode, which areadjacent to each other, of the optical intensity distribution within thereflector, and a center of the second layer is located at a secondposition corresponding to a midpoint between a node and an antinode,which are adjacent to each other, of the optical intensity distributionwithin the reflector.
 4. An optical device according to claim 1, whereinthe first layer has a thermal resistance smaller than a thermalresistance of the second layer.
 5. An optical device according to claim1, wherein the second layer comprises a binary material.
 6. An opticaldevice according to claim 5, wherein the first layer is an AlAs layer.7. An optical device according to claim 1, wherein a difference betweena lattice constant of a substrate of the optical device and a latticeconstant of the second layer is smaller than a difference between thelattice constant of the substrate and a lattice constant of the firstlayer.
 8. An optical device according to claim 7, wherein the firstlayer is an AlN layer.
 9. An optical device according to claim 7,wherein the second layer is a GaN layer.
 10. A vertical cavity surfaceemitting laser, comprising: a substrate; and two reflectors providedabove the substrate, wherein at least one of the two reflectorscomprises the optical device according to claim
 1. 11. An optical devicefor generating light of a wavelength λ, comprising: a multilayerreflector including a first layer having an optical thickness of (λ/4−t)and a second layer having an optical thickness of (λ/4+t); and a cavityincluding a spacer layer and an active layer, and having an opticalthickness of an integral multiple of λ/2, wherein the first layer has ahigher refractive index than that of the second layer, wherein the layerof the multilayer that is closest to the cavity is the first layer, anda layer having an optical thickness of (λ/4+t/2) is provided between themultilayer reflector and the cavity.
 12. An optical device according toclaim 11, wherein two or more pairs of the first layer and the secondlayer are laminated.
 13. An optical device according to claim 11,wherein a center of the first layer is located at a first positioncorresponding to a midpoint between a node and an antinode, which areadjacent to each other, of the optical intensity distribution within thereflector, and a center of the second layer is located at a secondposition corresponding to a midpoint between a node and an antinode,which are adjacent to each other, of the optical intensity distributionwithin the reflector.
 14. An optical device according to claim 11,wherein the first layer has a thermal resistance larger than a thermalresistance of the second layer.
 15. An optical device according to claim14, wherein the second layer comprises a binary material.
 16. An opticaldevice according to claim 15, wherein the second layer is an AlAs layer.17. An optical device according to claim 1, wherein a difference betweena lattice constant of a substrate of the optical device and a latticeconstant of the second layer is larger than a difference between thelattice constant of the substrate and a lattice constant of the firstlayer.
 18. A vertical cavity surface emitting laser, comprising: asubstrate; and two reflectors provided above the substrate; wherein atleast one of the two reflectors comprises the optical device accordingto claim 11.